1. Field of the Invention
The present invention relates to a inspection method of a semiconductor device such as a semiconductor integrated circuit and an apparatus for checking the semiconductor device, and more particularly to a wiring current observation and checking method for checking defects in current-carrying paths and the wiring system of wirings formed on a semiconductor integrated circuit chip, and an apparatus for the same.
2. Description of the Related Art
Concerning failure analysis and checking of a semiconductor device such as a semiconductor integrated circuit which is an objective of the present invention,. there have been disclosures, for example, in Japanese Patent Laid-Open No. 6-300824, "Inspection Method of Mutual wiring in Semiconductor Integrated Circuit and Apparatus for checking the Same", in Japanese Patent Application No. 7-025758, filed on Feb. 15, 1995, "Testing Method of Wirings on Semiconductor Integrated Circuit Chip and Apparatus for the Same".
FIG. 1 is a structure of a testing apparatus for testing semiconductor devices disclosed in these disclosed documents. An integrated circuit 112 as a tested sample is mounted on a sample stage 111. Laser beam 119 as a visible light emitted from a visible beam emission section 113 is made to be incident on a microscope section 114. The laser beam 119 made to be incident on the microscope section 114 is converged and irradiated onto a chip on the integrated circuit 112. A constant voltage source 115, a current variation detection section 117, and a testing pattern generating section 118 are connected to the sample stage 111. The testing pattern generating section 118 generates testing patterns to set the integrated circuit 112, onto which the laser beam 119 is irradiated, at a prescribed state. Each of these sections 115, 117, and 118 is electrically connected to corresponding pins formed in the integrated circuit 112.
The microscope section 114, the constant voltage source 115, the current variation detection section 117, and the testing pattern generating section 118 are connected to a system control/signal processing section 121 to control the total system and to process the acquired signal. The system control/signal processing section 121 is designed such that it operates a predetermined controlling and signal processing. An image display section 122 is formed of a CRT, and is connected to the system control/signal processing section 121. The image display section 122 is designed such that a current image or a defect image as a result of processing the acquired signal is displayed.
In this testing apparatus for testing semiconductor device, the laser beam as a visible light is irradiated onto the region of the integrated circuit 112 which is a detection objective, with the laser beam scanning on the region. The irradiated light may be electrons or ion beam instead of a laser beam. An increase in resistivity of the integrated circuit due to an increase in temperature caused by the irradiated light is detected as current variation using the current variation detection section 117. The current variations in the wirings are displayed, in synchronization with the scanning by the laser beam 119, on the image display section 122 as brightness variations detected in every irradiated position or as pseudo colors obtained by replacing the brightness with color. Thus, voids in the wirings and Si nodule, i.e. Si precipitates can be detected, and the current flowing through the wirings can be observed.
A principle of such detection and observation will be described briefly below. The current variation due to an increase in temperature at the time of irradiation of the laser beam onto the wirings on the semiconductor chip is expressed as .DELTA.I. When assuming that a constant voltage is applied to both ends of the wiring, the current variation .DELTA.I can be approximated using the following formula (1). EQU .DELTA.I.apprxeq.-(.DELTA.R/R)I (1)
In the formula (1), R denotes resistivity of the wiring when the laser beam is not irradiated onto the wiring, and .DELTA.R denotes a variation of the resistance of the wiring due to the irradiation of the laser beam onto the wiring. Furthermore, I denotes current flowing through the wiring when the laser beam is not irradiated onto the wiring.
As described above, if other conditions are unchanged, since the resistance R of the specified wiring to be observed is constant, the observation of the current variation .DELTA.I makes it possible to acquire the product of the variation of the resistance .DELTA.R and the current I. Furthermore, when the current I is made constant, the variation of the resistance .DELTA.R in each position of the wiring can be detected. In addition, the variation of the resistance .DELTA.R is constant, the value of the current I can be observed. Each of the above mentioned matters will be described individually.
(1) Observation of the Differences among the Variation of the Resistance related to the Positions of the Wiring: PA0 (2) Observation of the Current I: PA0 (1) When a laser beam or electron beam is irradiated onto a chip, electron/hole pairs are generated in a semiconductor substrate, so that a current is generated. When light is irradiated onto the chip, an OBIC (Optical Beam Induced Current) phenomenon appears, and when electrons are irradiated onto the chip, an EBIC (Electron Beam Induced Current) phenomenon appears. Only the OBIC phenomenon will be described below. The same thing can be said about the EBIC phenomenon. PA0 (2) In cases where an ion beam is used instead of the light beam, since the irradiated portions are sputtered, there is a problem that it is difficult to perform a non-destructive test.
This is disclosed as a detection method of defects such as voids and Si precipitates in Japanese Patent Laid Open No. 6-300824. Specifically, if laser beam conditions, materials of irradiated positions, and shapes of the wiring are same, the differences of the resistance variation of the wiring .DELTA.R among the detected positions in the wiring are generated due to the differences of the thermal conductivities depending on the detected positions. If there are voids and Si precipitates in the wiring, the thermal conductivities will be different. It is confirmed experimentally that the differences of the resistance variation of the wiring .DELTA.R can be observed by this effect. The voids and Si precipitates in the wiring are important as factors to determine reliability of the integrated circuit, so that this effect is important. An image observed for this purpose is called a defect image.
This is disclosed in Japanese Patent Application 7-025758. When aiming at the portion of the wiring having no defects, which is true for normally almost the whole wiring, the current I can be observed. To obtain an absolute value of the current I, it is essential to know the values of the resistance variation of the wiring .DELTA.R and the resistance R, but it is difficult to know these values. However, it is easy to know the existence of an abnormal current by comparing with a normal current. An image observed for this purpose is called a current image.
In the following description, phenomenon in which resistances of portions irradiated by the laser beam vary is simply called BIRCH (Beam Induced Resistance Change). When this BIRCH is used for a laser, electrons, and ions, this is called OBIRCH (Optical Beam Induced Resistance Change), EBIRCH, and IBIRCH respectively. Here, "O" denotes "Optical", "E" denotes "Electron", and "I" denotes "Ion".
A method called NB-OBIC (Non-bias Optical Beam Induced Current) makes use of thermoelectric effect exerted at heating of the laser beam, as a method using heating by means of beam. This method can be applied effectively to the detection of the voids and the like in the wiring, which has been disclosed in the document for the meeting of Applied Physics Society held in Autumn, 1994, 22a-ZP-10, p. 586, by Koyama et al. This NB-OBIC method differs from the BIRCH method only in that a voltage is not applied to the integrated circuit which is an objective to be detected. Other steps in the NB-OBIC method the same as those in the BIRCH method. Since the voltage is not applied to the integrated circuit so that no current flows, this NB-OBIC method can not be employed for observation of the current. A fundamental principle of the NB-OBIC method is as follows. When defects are present in a wiring system, a capability of thermal conduction vary near the defects, or thermal conduction state varies. Thus, a thermal gradient is produced near the defects when a laser beam is scanned so that thermoelectric power is produced. It is detected as a current.
There have been the following two severe problems in the foregoing conventional detection method and apparatus for the semiconductor devices. Application of the foregoing conventional method and apparatus to actual semiconductor products is obstructed by this fact.
An OBIC image appears in the form that it overlaps with the foregoing OBIRCH and NB-OBIC images. Normally, an OBIC signal is stronger than OBIRCH and NB-OBIC signals. A dynamic range in a current variation detection system is not wide enough to amplify both signals, so that the OBIRCH and NB-OBIC images are buried by the OBIC image and disappear. In case of a TEG (Test Element Group or Test Structure), it is possible to form a connection so that the OBIC image does not appear. However, in case of products, it is usually impossible. This is one of the severe obstacles to putting into practice the OBIRCH and NB-OBIC methods.